This invention addresses the problem of how to remotely position, orient, and clamp an object, which can be of virtually any size and shape within given limits, using a single fixturing device in a manner suitable to permit a machine to perform a task on the part. Such remote controlled reprogrammable fixturing devices will be required for use in automated factories, in particular those which use robots to load and unload parts. Specifically, the invention concerns a computer controlled workholding fixture with three programmable position degrees of freedom and one angular degree of freedom that are used to justify a prismatic part's location, and robotically changeable, disposable jaws, which can be machined to be of the proper shape required to hold a non-prismatic part. Currently, parts are held by dedicated fixtures which are assembled off-line by human operators. Some fixtures can be remotely controlled to clamp or release a part, but no presently available fixture can reconfigure itself to handle a wide variety of part types.
As industry becomes more automated, and factories are designed to operate "lights out" 24 hours per day, a need arises for a fixturing system that is as flexible as the machine tool it services. The increasing use of robots in flexible manufacturing systems also creates a need for fixtures which can be tended by robots. Similarly, a large portion of the high cost of prototype mechanisms is attributable to the setup time required to machine the parts.
A major part of the setup time is required to position, orient, and clamp the part with respect to the machine tool. The fixturing options presently available for use in automated manufacturing can be divided into two groups: those which are assembled on the machine tool, and those which are assembled on pallets which are then loaded onto the machine tool. Both types may use jigs or fixtures designed specifically for a part, or the fixture may be built up from basic building blocks (e.g. tombstones, clamps, vises); however, they consist of simple clamps and vises which require the skill of a machine operator to set them up. This requirement for a machine operator does not allow for 24 hour untended factory operation. If, however, the fixturing system could be electronically controlled, it would be possible to coordinate the actions of a robot, a machine tool, and a fixturing system to yield a workstation that could handle a variety of parts without requiring a human operator.
For high productivity, the machine tool should be continually operated (e.g. cutting metal). Thus, for all but high volume production runs, the fixturing system should be able to be changed as quickly as the cutting tools used on the machine. Additionally, like qualified tooling having dimensions carefully preset in the tooling room to ensure accuracy, the fixture must be able to hold a part in a known orientation and position with respect to the machine. The only way to accomplish the foregoing using present methods, however, is to use a palletized system which does not allow for untended operation of the factory as discussed above and which requires the transportation of large, heavy pallets from machine to machine a very inefficient, slow, and expensive process. If the proper fixturing system were available such that only the transportation of parts were necessary, efficiency and speed of machining operations could be significantly improved and costs substantially reduced.
Little work has been done in design of remotely controlled fixturing systems that can handle a wide variety of parts. Examples of the prior art include U.S. Pat. Nos. 4,437,654, and 4,251,066 to Chiappetti and Bowling, respectively, which describe replaceable jaws for manual vises. It should be noted that them described are fitted by sliding the jaws into various types of mechanical grooves, and then locking them in place with a bolt. The sliding of jaws into grooves can be difficult for a robot, and if the geometries shown were used in a computer controlled machining center which uses a spray stream of coolant, the jaw/vise interface would become contaminated and accuracy would be lost. Also, neither patent describes holding a blank jaw in the vise, and then using the machine tool to machine the desired shape into the jaw for holding an odd shaped part. Furthermore, the mechanisms described do not provide adequate positioning accuracy for the jaws if a blank jaw with a special shape was used to hold a part.
In another example of the prior art, U.S. Pat. No. 3,463,479 by Hennessey describes a grid of holes machined into the face of a vise jaw and into which pins can be manually inserted to act as variable position supports to hold a part; however, this system cannot be computer controlled and thus is not suitable for use in a computerized factory. Also, only discrete positioning is possible, levelling of non-flat parts cannot be accomplished.
Other examples of current fixturing methods include hold down clamps which are used primarily to hold plate type parts, or castings at their base. Hydraulic swing clamps are now widely available, and they can be arranged in a grid and controlled by electronically controlled solenoid valves. This type of arrangement, however, can only accomodate wide, flat parts or parts with hold down tabs.
Perhaps the other most common fixture in use today is a simple machine tool vise which may be hand or mechanically powered. To effectively use a vise, an operator uses parallels to set the height of a part. One edge is forced against a fixed anvil by a moving anvil (jaw), and the remaining reference needed to position the part is obtained using a stop, or an edge finder.
In contrast to these simple devices which must be set up by a human operator, the ideal remote controlled, reconfigurable fixturing system must be able to position, orient, and clamp two categories of parts: prismatic and non-prismatic. The former are easily handled by a vise type system, while the latter may require special jigs to accomodate various geometric shapes. The key to all fixturing, however, is how to present the part to the machine tool and how to maintain a holding/clamping force during the machining process.
Clearly one fixture cannot be called upon to handle all fixturing needs of a flexible manufacturing system, but a hierarchical mechanical system, which provides a base to which specialized fixtures can be attached, could handle most fixturing needs. Note that in a hierarchical computer controlled flexible manufacturing facility, "idle" machines could be directed to manufacture fixture attachments using downloaded numerical control programs generated from a CAD/CAM generated model of the part to be fixtured. Since a robot cannot compute with human dexterity when it comes to bolting things down, advantage will be taken of a hierarchical computer controlled facility's ability to schedule jobs and download parts programs to workcells which use a remote controlled flexible fixturing system to manufacture disposable fixturing attachments on line. This will require more metal to be cut, but fixturing inventories and setup time will be reduced. This will allow for the automation of small batch jobs, and will allow them to be run overnight with the lights out.